Many hope the UK's economy will flourish because of the close relationship between London and Beijing.

Deals worth more than £40bn ($56bn) were signed when Chinese President Xi Jinping visited the UK last October.

During that visit, Mr Xi made his views clear: China supports a "united EU".

"China definitely wants to sell more Chinese products into the European market, but they'd better do it from the eurozone really," says Philippe Le Corre, a visiting fellow at the Brookings Institution in Washington.

London would be less attractive to investors if it could offer only the UK market of 65 million people, rather than the 500 million across the EU, he says.

"London would not lose its role as trader of renminbi but in terms of consumer products, big industries, even tourism, I believe London would be affected."

But it's not all about money.

301 Moved Permanently

Moved Permanently

EU referendum issues guide: Explore the argumentshttp://www.bbc.co.uk/news/uk-politics-eu-referendum-36027205https://www.bbc.co.uk/news/special/2016/newsspec_13606/content/iframe/english/index.inc.app.htmlExplore all the issuesChoose an issue:What both sides are sayingAll issuesMain viewsShare this pageEU referendum issues guideWhat the leave and remain sides are saying in the #EUref campaign

For Mr Xi, the EU referendum also carries political concerns.

During his time in power, over almost four years, he's fostered closer ties with London - a counterbalance, perhaps, to China's rockier relations elsewhere.

If the UK leaves the EU, Mr Xi's judgement could be called into question.

"There has been some criticism that he has been spending too much attention on ceremonial matters and diplomacy without concern for the slowing Chinese economy at home," says Yu Jie, the China programme manager at the LSE Ideas think tank in London.

"So, if the UK leaves the EU, it won't only undermine the Chinese relationship with the UK. It will also undermine Xi Jinping's image as a steady leader because he's betting on the wrong horse."

Image copyrightGetty ImagesImage caption
Could that time building a rapport with Mr Cameron have been wasted?

Chinese entrepreneurs might also come to feel that they've bet on the wrong horse. Over tea at his private members' club, Mr Zai admits to worries.

"I'm just afraid if we leave the EU, it's going to be like an earthquake, like a huge earthquake. To be honest, I have no idea what's going to happen. Personally, I'm quite nervous."

Nerves in London stretching all the way to the halls of power in Beijing.

Some might argue that Chinese concerns shouldn't matter and British voters will make their own decisions on EU membership.

But those who have invested financial and political capital in a closer relationship between China and the UK might be heading for a few sleepless nights.